happstack-server-7.1.7: Web related tools and services.

Safe Haskell | None

Happstack.Server.Internal.Monads

Description

This module defines the Monad stack used by Happstack. You mostly don't want to be looking in here. Look in Happstack.Server.Monads instead.

Synopsis

- type Web a = WebT IO a
- type ServerPart a = ServerPartT IO a
- newtype ServerPartT m a = ServerPartT {
  - unServerPartT :: ReaderT Request (WebT m) a
  }
- runServerPartT :: ServerPartT m a -> Request -> WebT m a
- withRequest :: (Request -> WebT m a) -> ServerPartT m a
- anyRequest :: Monad m => WebT m a -> ServerPartT m a
- mapServerPartT :: (UnWebT m a -> UnWebT n b) -> ServerPartT m a -> ServerPartT n b
- mapServerPartT' :: (Request -> UnWebT m a -> UnWebT n b) -> ServerPartT m a -> ServerPartT n b
- class Monad m => ServerMonad m where
  - askRq :: m Request
  - localRq :: (Request -> Request) -> m a -> m a
- data SetAppend a
  - = Set a
  - | Append a
- extract :: SetAppend t -> t
- type FilterFun a = SetAppend (Dual (Endo a))
- unFilterFun :: FilterFun a -> a -> a
- filterFun :: (a -> a) -> FilterFun a
- newtype FilterT a m b = FilterT {
  - unFilterT :: WriterT (FilterFun a) m b
  }
- class Monad m => FilterMonad a m | m -> a where
  - setFilter :: (a -> a) -> m ()
  - composeFilter :: (a -> a) -> m ()
  - getFilter :: m b -> m (b, a -> a)
- ignoreFilters :: FilterMonad a m => m ()
- newtype WebT m a = WebT {
  - unWebT :: ErrorT Response (FilterT Response (MaybeT m)) a
  }
- type UnWebT m a = m (Maybe (Either Response a, FilterFun Response))
- class Monad m => WebMonad a m | m -> a where
  - finishWith :: a -> m b
- escape :: (WebMonad a m, FilterMonad a m) => m a -> m b
- escape' :: (WebMonad a m, FilterMonad a m) => a -> m b
- ununWebT :: WebT m a -> UnWebT m a
- mkWebT :: UnWebT m a -> WebT m a
- mapWebT :: (UnWebT m a -> UnWebT n b) -> WebT m a -> WebT n b
- localContext :: Monad m => (WebT m a -> WebT m' a) -> ServerPartT m a -> ServerPartT m' a
- multi :: Monad m => [ServerPartT m a] -> ServerPartT m a
- debugFilter :: (MonadIO m, Show a) => ServerPartT m a -> ServerPartT m a
- outputTraceMessage :: String -> a -> a
- mkFailMessage :: (FilterMonad Response m, WebMonad Response m) => String -> m b
- failHtml :: String -> String
- escapeString :: String -> String

# Documentation

type Web a = WebT IO a

An alias for `WebT` when using `IO`.

type ServerPart a = ServerPartT IO a

An alias for ``ServerPartT` `IO``

newtype ServerPartT m a

`ServerPartT` is a rich, featureful monad for web development.

see also: `simpleHTTP`, `ServerMonad`, `FilterMonad`, `WebMonad`, and `HasRqData`

Constructors

ServerPartT |

Fields

unServerPartT :: ReaderT Request (WebT m) a

Instances

MonadTrans ServerPartT |
MonadTransControl ServerPartT |
MonadBase b m => MonadBase b (ServerPartT m) |
MonadBaseControl b m => MonadBaseControl b (ServerPartT m) |
(Monad m, MonadError e m) => MonadError e (ServerPartT m) |
(Monad m, MonadReader r m) => MonadReader r (ServerPartT m) |
(Monad m, MonadState s m) => MonadState s (ServerPartT m) |
(Monad m, MonadWriter w m) => MonadWriter w (ServerPartT m) |
Monad m => WebMonad Response (ServerPartT m) |
Monad m => FilterMonad Response (ServerPartT m) |
Monad m => Monad (ServerPartT m) |
Functor m => Functor (ServerPartT m) |
Monad m => MonadPlus (ServerPartT m) |
(Monad m, Functor m) => Applicative (ServerPartT m) |
(Functor m, MonadPlus m) => Alternative (ServerPartT m) |
MonadIO m => MonadIO (ServerPartT m) |
Monad m => ServerMonad (ServerPartT m) |
MonadIO m => HasRqData (ServerPartT m) |
(Functor m, Monad m, MonadPlus m, MonadIO m) => Happstack (ServerPartT m) |
Monad m => Monoid (ServerPartT m a) |

runServerPartT :: ServerPartT m a -> Request -> WebT m a

Particularly useful when combined with `runWebT` to produce a `m (`Maybe` `Response`)` from a `Request`.

withRequest :: (Request -> WebT m a) -> ServerPartT m a

function for lifting WebT to ServerPartT

NOTE: This is mostly for internal use. If you want to access the `Request` in user-code see `askRq` from `ServerMonad`.

     do request <- askRq
        ...

anyRequest :: Monad m => WebT m a -> ServerPartT m a

A constructor for a `ServerPartT` when you don't care about the request.

NOTE: This is mostly for internal use. If you think you need to use it in your own code, you might consider asking on the mailing list or IRC to find out if there is an alternative solution.

mapServerPartT :: (UnWebT m a -> UnWebT n b) -> ServerPartT m a -> ServerPartT n b

Apply a function to transform the inner monad of ``ServerPartT` m`.

Often used when transforming a monad with `ServerPartT`, since `simpleHTTP` requires a ``ServerPartT` `IO` a`. Refer to `UnWebT` for an explanation of the structure of the monad.

Here is an example. Suppose you want to embed an `ErrorT` into your `ServerPartT` to enable `throwError` and `catchError` in your `Monad`.

     type MyServerPartT e m a = ServerPartT (ErrorT e m) a

Now suppose you want to pass `MyServerPartT` into a function that demands a ``ServerPartT` `IO` a` (e.g. `simpleHTTP`). You can provide the function:

       unpackErrorT :: (Monad m, Show e) => UnWebT (ErrorT e m) a -> UnWebT m a
       unpackErrorT et = do
          eitherV <- runErrorT et
          return $ case eitherV of
              Left err -> Just (Left $ toResponse $
                                       "Catastrophic failure " ++ show err
                               , filterFun $ \r -> r{rsCode = 500})
              Right x -> x

With `unpackErrorT` you can now call `simpleHTTP`. Just wrap your `ServerPartT` list.

      simpleHTTP nullConf $ mapServerPartT unpackErrorT (myPart `catchError` myHandler)

Or alternatively:

      simpleHTTP' unpackErrorT nullConf (myPart `catchError` myHandler)

Also see `spUnwrapErrorT` for a more sophisticated version of this function.

mapServerPartT' :: (Request -> UnWebT m a -> UnWebT n b) -> ServerPartT m a -> ServerPartT n b

A variant of `mapServerPartT` where the first argument also takes a `Request`. Useful if you want to `runServerPartT` on a different `ServerPartT` inside your monad (see `spUnwrapErrorT`).

class Monad m => ServerMonad m where

The `ServerMonad` class provides methods for reading or locally modifying the `Request`. It is essentially a specialized version of the `MonadReader` class. Providing the unique names, `askRq` and `localRq` makes it easier to use `ServerPartT` and `ReaderT` together.

Methods

askRq :: m Request

localRq :: (Request -> Request) -> m a -> m a

Instances

Monad m => ServerMonad (ServerPartT m) |
(Error e, ServerMonad m) => ServerMonad (ErrorT e m) |
ServerMonad m => ServerMonad (ReaderT r m) |
ServerMonad m => ServerMonad (StateT s m) |
(ServerMonad m, Monoid w) => ServerMonad (WriterT w m) |
(ServerMonad m, Monoid w) => ServerMonad (RWST r w s m) |

data SetAppend a

A monoid operation container. If `a` is a monoid, then `SetAppend` is a monoid with the following behaviors:

      Set    x `mappend` Append y = Set    (x `mappend` y)
      Append x `mappend` Append y = Append (x `mappend` y)
      _        `mappend` Set y    = Set y

A simple way of summarizing this is, if the right side is `Append`, then the right is appended to the left. If the right side is `Set`, then the left side is ignored.

Constructors

Set a |
Append a |

Instances

Functor SetAppend |
Eq a => Eq (SetAppend a) |
Show a => Show (SetAppend a) |
Monoid a => Monoid (SetAppend a) |

extract :: SetAppend t -> t

Extract the value from a `SetAppend`. Note that a `SetAppend` is actually a `CoPointed` from: http://hackage.haskell.org/packages/archive/category-extras/latest/doc/html/Control-Functor-Pointed.html But lets not drag in that dependency. yet...

type FilterFun a = SetAppend (Dual (Endo a))

`FilterFun` is a lot more fun to type than ``SetAppend` (`Dual` (`Endo` a))`.

unFilterFun :: FilterFun a -> a -> a

filterFun :: (a -> a) -> FilterFun a

turn a function into a `FilterFun`. Primarily used with `mapServerPartT`

newtype FilterT a m b

Constructors

FilterT |

Fields

unFilterT :: WriterT (FilterFun a) m b

Instances

MonadBase b m => MonadBase b (FilterT a m) |
MonadBaseControl b m => MonadBaseControl b (FilterT a m) |
Monad m => FilterMonad a (FilterT a m) |
MonadTrans (FilterT a) |
MonadTransControl (FilterT a) |
Monad m => Monad (FilterT a m) |
Functor m => Functor (FilterT a m) |
Applicative m => Applicative (FilterT a m) |
MonadIO m => MonadIO (FilterT a m) |

class Monad m => FilterMonad a m | m -> a where

A set of functions for manipulating filters.

`ServerPartT` implements `FilterMonad` `Response` so these methods are the fundamental ways of manipulating `Response` values.

Methods

setFilter :: (a -> a) -> m ()

Ignores all previous alterations to your filter

As an example:

     do
       composeFilter f
       setFilter g
       return "Hello World"

The ``setFilter` g` will cause the first ``composeFilter` f` to be ignored.

composeFilter :: (a -> a) -> m ()

Composes your filter function with the existing filter function.

getFilter :: m b -> m (b, a -> a)

Retrieves the filter from the environment.

Instances

Monad m => FilterMonad Response (WebT m) |
Monad m => FilterMonad Response (ServerPartT m) |
(FilterMonad res m, Monoid w) => FilterMonad res (WriterT w m) |
FilterMonad res m => FilterMonad res (StateT s m) |
FilterMonad res m => FilterMonad res (ReaderT r m) |
Monad m => FilterMonad a (FilterT a m) |
(FilterMonad res m, Monoid w) => FilterMonad res (RWST r w s m) |

ignoreFilters :: FilterMonad a m => m ()

Resets all your filters. An alias for ``setFilter` `id``.

newtype WebT m a

The basic `Response` building object.

Constructors

WebT |

Fields

unWebT :: ErrorT Response (FilterT Response (MaybeT m)) a

Instances

MonadTrans WebT |
MonadTransControl WebT |
MonadBase b m => MonadBase b (WebT m) |
MonadBaseControl b m => MonadBaseControl b (WebT m) |
MonadError e m => MonadError e (WebT m) |
MonadReader r m => MonadReader r (WebT m) |
MonadState st m => MonadState st (WebT m) |
MonadWriter w m => MonadWriter w (WebT m) |
Monad m => WebMonad Response (WebT m) |
Monad m => FilterMonad Response (WebT m) |
Monad m => Monad (WebT m) |
Functor m => Functor (WebT m) |
Monad m => MonadPlus (WebT m) |
(Monad m, Functor m) => Applicative (WebT m) |
(Functor m, MonadPlus m) => Alternative (WebT m) |
MonadIO m => MonadIO (WebT m) |
Monad m => Monoid (WebT m a) |

type UnWebT m a = m (Maybe (Either Response a, FilterFun Response))

`UnWebT` is almost exclusively used with `mapServerPartT`. If you are not using `mapServerPartT` then you do not need to wrap your head around this type. If you are -- the type is not as complex as it first appears.

It is worth discussing the unpacked structure of `WebT` a bit as it's exposed in `mapServerPartT` and `mapWebT`.

A fully unpacked `WebT` has a structure that looks like:

     ununWebT $ WebT m a :: m (Maybe (Either Response a, FilterFun Response))

So, ignoring `m`, as it is just the containing `Monad`, the outermost layer is a `Maybe`. This is `Nothing` if `mzero` was called or ``Just` (`Either` `Response` a, `SetAppend` (`Endo` `Response`))` if `mzero` wasn't called. Inside the `Maybe`, there is a pair. The second element of the pair is our filter function ``FilterFun` `Response``. ``FilterFun` `Response`` is a type alias for ``SetAppend` (`Dual` (`Endo` `Response`))`. This is just a wrapper for a ``Response` -> `Response`` function with a particular `Monoid` behavior. The value

      Append (Dual (Endo f))

Causes `f` to be composed with the previous filter.

      Set (Dual (Endo f))

Causes `f` to not be composed with the previous filter.

Finally, the first element of the pair is either ``Left` `Response`` or ``Right` a`.

Another way of looking at all these pieces is from the behaviors they control. The `Maybe` controls the `mzero` behavior. ``Set` (`Endo` f)` comes from the `setFilter` behavior. Likewise, ``Append` (`Endo` f)` is from `composeFilter`. ``Left` `Response`` is what you get when you call `finishWith` and ``Right` a` is the normal exit.

An example case statement looks like:

      ex1 webt = do
        val <- ununWebT webt
        case val of
            Nothing -> Nothing  -- this is the interior value when mzero was used
            Just (Left r, f) -> Just (Left r, f) -- r is the value that was passed into "finishWith"
                                                 -- f is our filter function
            Just (Right a, f) -> Just (Right a, f) -- a is our normal monadic value
                                                   -- f is still our filter function

class Monad m => WebMonad a m | m -> a where

`WebMonad` provides a means to end the current computation and return a `Response` immediately. This provides an alternate escape route. In particular it has a monadic value of any type. And unless you call ``setFilter` `id`` first your response filters will be applied normally.

Extremely useful when you're deep inside a monad and decide that you want to return a completely different content type, since it doesn't force you to convert all your return types to `Response` early just to accommodate this.

see also: `escape` and `escape'`

Methods

finishWith

Arguments

:: a | value to return (For `ServerPart`, `a` will always be the type `Response`)
-> m b |

Instances

Monad m => WebMonad Response (WebT m) |
Monad m => WebMonad Response (ServerPartT m) |
(WebMonad a m, Monoid w) => WebMonad a (WriterT w m) |
WebMonad a m => WebMonad a (StateT s m) |
WebMonad a m => WebMonad a (ReaderT r m) |
(WebMonad a m, Monoid w) => WebMonad a (RWST r w s m) |

escape :: (WebMonad a m, FilterMonad a m) => m a -> m b

Used to ignore all your filters and immediately end the computation. A combination of `ignoreFilters` and `finishWith`.

escape' :: (WebMonad a m, FilterMonad a m) => a -> m b

An alternate form of `escape` that can be easily used within a do block.

ununWebT :: WebT m a -> UnWebT m a

For when you really need to unpack a `WebT` entirely (and not just unwrap the first layer with `unWebT`).

mkWebT :: UnWebT m a -> WebT m a

For wrapping a `WebT` back up. ``mkWebT` . `ununWebT` = `id``

mapWebT :: (UnWebT m a -> UnWebT n b) -> WebT m a -> WebT n b

See `mapServerPartT` for a discussion of this function.

localContext :: Monad m => (WebT m a -> WebT m' a) -> ServerPartT m a -> ServerPartT m' a

This is kinda like a very oddly shaped `mapServerPartT` or `mapWebT`. You probably want one or the other of those.

multi :: Monad m => [ServerPartT m a] -> ServerPartT m a

Deprecated: Use msum instead

Deprecated: use `msum`.

debugFilter :: (MonadIO m, Show a) => ServerPartT m a -> ServerPartT m a

Deprecated: This function appears to do nothing.

What is this for, exactly? I don't understand why `Show a` is even in the context Deprecated: This function appears to do nothing at all. If it use it, let us know why.

outputTraceMessage :: String -> a -> a

mkFailMessage :: (FilterMonad Response m, WebMonad Response m) => String -> m b

failHtml :: String -> String

escapeString :: String -> String